Sprocket and manufacturing method thereof

ABSTRACT

A sprocket made of low carbon steel of not more than 0.25 wt % in the carbon content comprising teeth portion which are quenched to a hardness close to the theoretical maximum quenching hardness of 35-55 Rockwell hardness C, and an inside diameter mounting hole.

TECHNICAL FIELD

The present invention relates to a sprocket made of low carbon steel,the carbon content of which is not more than 0.25 wt %, and amanufacturing method thereof.

BACKGROUND ART

Conventionally, carbon steel of which the carbon content is 0.40 to 0.50wt % (referred to as medium carbon steel in this specificationhereinafter), for example, carbon steel of S45C stipulated in JIS isused for a sprocket to transmit power in a motorcycle.

The process of manufacturing a sprocket includes, for example, 15processes of (a) to (j) as shown in FIGS. 7 and 8. When the teethportion of a sprocket undergo a heat-treatment in the teeth portionquenching process shown in FIG. 8(g), hardness of the teeth portion isset at 35 to 50 of the Rockwell hardness C, so that the anti-abrasionproperty can be enhanced.

On the other hand, there is provided a sprocket made of carbon steel,the carbon content of which is not more than 0.25 wt % (referred to aslow carbon steel in this specification hereinafter), for example, lowcarbon steel of SPH to S20C stipulated in JIS. The above sprocket isused in some regions because a plate of low carbon steel is inexpensive.

It is well known that even the hardness of low carbon steel is somewhatincreased when it is heated to a temperature not lower than the A₃transformation point and then dipped in water so as to be quenched.There is provided a sprocket, the teeth portion of which undergo a heattreatment so that the hardness can be increased. However, when thematter is considered from an essential viewpoint, it is impossible toquench low carbon steel, and it is difficult to sufficiently increasethe hardness of low carbon steel.

In the case of a sprocket made of medium carbon steel, it is possible tosufficiently increase the hardness of the teeth portion. Therefore, thesprocket made of medium carbon steel is excellent in the anti-abrasionproperty. However, the material cost of medium carbon steel is high, andfurther distortions and cracks tend to occur in the process of pressforming since the hardness is high. Due to the above characteristic ofmedium carbon steel, it is difficult to manufacture a sprocket mainly bymeans of press forming. Therefore, it is necessary to conduct machiningin the five essential manufacturing processes including an outsidediameter cutting process, inside diameter cutting process, teeth cuttingprocess, mounting hole drilling process and mounting hole chamferingprocess. Since the number of processes is increased, the manufacturingcost is high. Further, a large quantity of chips are produced in theprocess of machining, and it is complicated to remove the producedchips. Furthermore, due to the abrasion of a cutting tool, errors tendto occur in the manufacturing process. Accordingly, it is necessary topay close attention in order to stabilize the machining accuracy whenthe sprocket is manufactured.

On the other hand, in the case of a sprocket made of low carbon steel,the following advantages are provided. The material cost is reduced. Dueto the characteristic of low carbon steel, no distortions are generatedin the process of press forming. Therefore, it is not necessary tomachine after-processing such as finishing. Accordingly, it is possibleto manufacture a sprocket mainly by means of press forming. Further, themanufacturing process of a sprocket made of low carbon steel is simplerthan that of a sprocket made of medium carbon steel. Therefore, themanufacturing cost can be greatly reduced. In spite of the aboveadvantages, the sprocket made of low carbon steel is disadvantageous inthat it is impossible to provide a sufficiently high hardness andfurther the anti-abrasion property is greatly deteriorated.

DISCLOSURE OF THE INVENTION

The present invention has been accomplished by the inventors whodiscovered that even the hardness of low carbon steel can be increasedto a value close to the theoretical maximum quenching hardness when itis cooled more quickly than it is cooled being dipped in water. It is anobject of the present invention to provide a sprocket to be manufacturedat low cost, the anti-abrasion property of which is high. Also, it is anobject of the present invention to provide a method of manufacturing thesprocket.

According to the first aspect, the present invention is to provide asprocket made of low carbon steel, the carbon content of which is notmore than 0.25 wt %, and the teeth portion of which are quenched to ahardness close to the theoretical maximum quenching hardness.

According to the second aspect, the present invention is to provide asprocket in which the teeth portion are quenched to 35 to 55 of theRockwell hardness C.

According to the third aspect, the present invention is to provide asprocket in which the carbon content is set at 0.10 to 0.20 wt %.

According to the fourth aspect, the present invention is to provide asprocket in which a ratio of the shearing surface to the innercircumferential surface of the inside diameter mounting hole is not lessthan 50%.

According to the fifth aspect, the present invention is to provide amethod of manufacturing a sprocket comprising the steps of: machining alow carbon steel plate, the carbon content of which not more than 0.25wt %, into a shape of a sprocket; and quenching the teeth portion of themachined sprocket heating to a temperature not lower than the A₃transformation point and quickly cooling to a temperature lower than500° C. within 0.5 sec so as to quench the teeth portion to a hardnessclose to the theoretical maximum quenching hardness.

According to the sixth aspect, the present invention is to provide amethod of manufacturing a sprocket in which the carbon content of a lowcarbon steel plate is set at 0.10 to 0.20 wt %.

According to the seventh aspect, the present invention is to provide amethod of manufacturing a sprocket in which the teeth portion arequenched to 35 to 55 of the Rockwell hardness C.

According to the eighth aspect, the present invention is to provide amethod of manufacturing a sprocket in which the teeth portion of themachined sprocket are heated to a temperature not lower than the A₃transformation point and watered by jet simultaneously with orimmediately after the stoppage of heating in the quenching process so asto quench the teeth portion.

According to the ninth aspect, the present invention is to provide amethod of manufacturing a sprocket in which the teeth portion of themachined sprocket are heated to a temperature not lower than the A₃transformation point and watered by jet immediately before the stoppageof heating in the quenching process so as to quench the teeth portion.

According to the tenth aspect, the present invention is to provide amethod of manufacturing a sprocket in which a shape of the sprocket isformed from a low carbon steel plate only by means of press forming inthe manufacturing process.

According to the eleventh aspect, the present invention is to provide amethod of manufacturing a sprocket in which at least the teeth portionof the sprocket are machined in the manufacturing process.

According to the twelfth aspect, the present invention is to provide amethod of manufacturing a sprocket in which the teeth portion of thesprocket are heated by a high-frequency induction heating device in thequenching process.

According to the present invention, a sprocket is made of low carbonsteel, the carbon content of which is not more than 0.25 wt %, and theteeth portion of this sprocket are quenched to a hardness close to thetheoretical maximum quenching hardness. Accordingly, while the sprocketis composed of inexpensive low carbon steel, the anti-abrasion propertyof the teeth portion of the sprocket can be enhanced as highly aspossible.

In this case, when a sprocket is made of low carbon steel, the carboncontent of which is 0.10 to 0.20 wt %, the sprocket can be manufacturedfrom a common inexpensive material while maintaining the anti-abrasionproperty of the teeth portion to be sufficiently high. When the teethportion are quenched to 35 to 55 of the Rockwell hardness C, it ispossible to maintain the anti-abrasion property of the teeth portion tobe sufficiently high. Further, when a ratio of the shearing surface tothe inner circumferential surface of the inside diameter mounting holeis set at a value not less than 50%, the sprocket can be highlyaccurately assembled to a rotary shaft attached to the inside diametermounting hole.

According to the method of manufacturing a sprocket of the presentinvention, first, in the manufacturing process, a low carbon steel plateof which the carbon content is not more than 0.25% is formed into ashape of the sprocket, and in the next quenching process, the teethportion of the sprocket are heated to a temperature not lower than theA₃ transformation point and then quickly cooled to a temperature nothigher than 500° C. within 0.5 sec. In this way, the sprocket can beprovided. When the teeth portion are quickly cooled and quenched from atemperature not lower than the A₃ transformation point to a temperaturenot higher than 500° C. within 0.5 sec, the Rockwell hardness C of theteeth portion can be set at a value close to the theoretical maximumquenching hardness.

In this case, when the carbon content of a low carbon steel plate is setat 0.10 to 0.20 wt %, the sprocket can be manufactured from a commoninexpensive material while maintaining the anti-abrasion property of theteeth portion to be sufficiently high. When the teeth portion arequenched to 35 to 55 of the Rockwell hardness C, it is possible tomaintain the anti-abrasion property of the teeth portion to besufficiently high. Further, when the teeth portion of the sprocket areheated to a temperature not lower than the A₃ transformation point andwatered by jet so as to be quenched simultaneously with or immediatelyafter the stoppage of heating, it is possible to quickly cool and quenchthe teeth portion to a temperature not higher than 500° C. within 0.5sec. When the teeth portion are watered by jet and cooled immediatelybefore the stoppage of heating, it is possible to prevent the occurrenceof a gentle drop of temperature which is caused in a period of time fromthe stoppage of heating to the start of jet-watering the teeth portion.Accordingly, it is possible to further increase the cooling speed of theteeth portion to be quenched.

Since the sprocket is made of low carbon steel, the carbon content ofwhich is low, it is possible to form a low carbon steel sheet into ashape of the sprocket only by means of press forming such as fineblanking. Accordingly, while the manufacturing process of the sprocketis simplified, a highly accurate sprocket of high quality can bemanufactured.

Further, when the teeth portion of the sprocket are heated by thehigh-frequency induction heating device in the quenching process, onlythe teeth portion can be quickly heated to a predetermined temperature.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a plan view of the sprocket.

FIG. 2 is a cross-sectional view taken on line II--II in FIG. 1.

FIGS. 3(a) through 3(i) are manufacturing process drawing of thesprocket.

FIG. 4 is a schematic illustration of the high-frequency inductionheating device.

FIG. 5 is a diagram showing the quenching hardness and others.

FIG. 6 is a continuous cooling transformation diagram of low carbonsteel, the carbon content of which is 0.13 wt %.

FIGS. 7(a) through 7(f) are manufacturing process drawing of theconventional sprocket.

FIGS. 8(g) through 8(j) are a manufacturing process drawing of theconventional sprocket.

BEST MODE FOR PRACTICING INVENTION

Referring to the accompanying drawings, an embodiment of the presentinvention will be explained as follows.

As shown in FIG. 1, the sprocket 1 includes: an annular boss 2 formed inthe center; an annular rim 3 formed on the outer circumference; aplurality of teeth 4 formed on the outer circumference of the rim 3;four arms 5 connecting the boss 2 with the rim 3; an inside diametermounting hole 6 through which an output shaft of an engine is inserted;four attaching holes 7 to which the bolts for fixing the sprocket 1 tothe output shaft are attached; and four pattern holes 8 for thereduction of the weight. Although the above common sprocket 1 is used inthis embodiment, sprockets of different structures may be adopted.

The sprocket 1 is made of low carbon steel, the carbon content of whichis not higher than 0.25 wt %, and preferably 0.10 to 0.2 wt %.Concerning the upper limit of the carbon content, when the upper limitis set at a value higher than 0.25 wt %, the material cost is highalthough a sufficiently high hardness can be provided. Accordingly, theupper limit is set at a value not higher than 0.25 wt %. In order toreduce the material cost as low as possible, it is preferable to set theupper limit at a value not higher than 0.2 wt %. Concerning the lowerlimit of the carbon content, when the carbon content is reduced to avalue lower than 0.10 wt % as shown in FIG. 5, even the theoreticalmaximum quenching hardness is reduced to a value not higher than 35 ofthe Rockwell hardness C, and it is impossible to provide a sufficientlyhigh anti-abrasion property. Therefore, the lower limit is set at avalue not lower than 0.10 wt %.

As described above, the carbon content of the sprocket 1 is low, it ispossible to manufacture the sprocket 1 through a manufacturing processmainly composed of press forming. As shown in FIG. 2, a ratio 100×(t/T)of the shearing surface to the inner circumferential surface of theinside diameter mounting hole 6 is set at a value not less than 50%, sothat the assembling accuracy with respect to the output shaft can beenhanced.

Next, referring to the manufacturing process diagram shown in FIG. 3,the manufacturing process of the sprocket 1 will be explained asfollows.

First, in the outside diameter finish punching process shown in FIG.3(a), a low carbon steel plate, the carbon content of which is 0.10 to0.25 wt %, is punched, and the first disk-shaped step 11 is made.

Next, in the inside diameter core punching process shown in FIG. 3(b), acenter of the first step 11 is punched, and the second step 12 is made.

Next, in the pattern hole punching process shown in FIG. 3(c), patternholes 8 are punched on the second step 12 by means of press forming, andthe third step 13 is made.

Next, in the inside diameter mounting hole finish punching process shownin FIG. 3(d), a center of the step is punched by means of press forming,and the inside diameter mounting hole 6 is formed, and at the same time,four attaching holes 7 are punched, and the fourth step 14 is made. Whena clearance between the punch and the die is set at a value not higherthan 5% at this time, punching is conducted in such a manner that theratio of the shearing surface to the inner circumferential surface ofthe inside diameter mounting hole 6 is maintained at a value not lowerthan 50%.

Next, in the teeth cutting process shown in FIG. 3(e), a plurality ofteeth 4 are formed on the outer circumference of the fourth step 14 bymeans of machining, and the fifth step 15 is made. In this connection,this teeth cutting process may be conducted by means of press forming.In the case of press forming, it is preferable to provide a burrremoving process after the teeth cutting process.

Next, in the teeth quenching process shown in FIG. 3(f), the teeth 4 areheated to a temperature not lower than the A₃ transformation point (forexample, not lower than 870° C.) with a high-frequency induction heatingdevice 21 described later, and simultaneously with the stoppage ofheating, the teeth 4 are watered by jet so that the teeth 4 can bequenched. In this way, the sixth step 16 is obtained. In thisconnection, the teeth 4 may be watered by jet immediately after thestoppage of heating so that the teeth 4 can be quenched.

At this time, the teeth 4 are quickly cooled to a temperature not higherthan 500° C. within 0.5 sec, and the Rockwell hardness C of the teeth isadjusted to a value in a range from 35 to 55 which is close to thetheoretical maximum hardness.

Now, referring to the continuous cooling transformation diagram shown inFIG. 6, the cooling speed of the teeth 4 will be briefly explainedbelow. Concerning the low carbon steel, the carbon content of which is0.13 wt %, in order to provide the Vickers hardness Hv 358 (in the caseof Rockwell hardness C, HRC≈37), it is necessary to quickly cool the lowcarbon steel from about 720° C. to 500° C. within 0.5 sec. In order toprovide 35 to 55 of the Rockwell hardness C, it is necessary to quicklycool the low carbon steel from about 720° C. to a temperature not higherthan 500° C. within 0.5 sec.

As another method of quickly cooling to a temperature not higher than500° C. within 0.5 sec, the teeth 4 may be watered by jet immediatelybefore the stoppage of heating conducted by the high-frequency inductionheating device 21. In this case, the occurrence of a gentle temperaturedrop caused in a period of time from the stoppage of heating to thestart of jet-watering the teeth 4 is prevented, so that the teeth 4 canbe cooled and quenched more quickly. Alternatively, the sprocket 1 maybe dipped and agitated in a solution into which a depressant of freezingpoint such as salt is added.

As shown in FIG. 4, the high-frequency induction heating device 21 has acommon structure including a substantially annular hollow induction coil22 made of copper which surrounds the sprocket 1. There are formed aplurality of jet holes 23 on the inner circumferential surface of theinduction coil 22, and the jet holes 23 are directed to the center ofthe induction coil 22. When the induction coil 21 is energized with acurrent of AC, the frequency of which is 30 to 300 KHz, under thecondition that the sprocket 1 is set inside the induction coil 21, onlythe teeth 4 of the sprocket 1 can be quickly heated by means ofhigh-frequency induction heating. When pressurized water is suppliedinside the induction coil 21, water can be jetted out from the pluralityof jet holes 23 to the teeth 4. Due to the foregoing, the teeth 4 can bequickly cooled. In this connection, another quenching method in whichthe induction heating coil is used will be described as follows. Underthe condition that the induction coil and the sprocket 1 are dipped inwater, the teeth 4 of the sprocket 1 are heated to a temperature notlower than the A₃ transformation point, and then electric power supplyto the induction coil is stopped, so that the sprocket 1 can be quicklycooled.

Next, in the surface treatment process shown in FIG. 3(i), the sixthstep 16, which has already been quenched, is subjected to plating orcoating. In this way, the sprocket 1 can be provided. However, in thecase where the sixth step 16 is not subjected to plating or coating,this process can be omitted.

As described above, the sprocket 1 can be manufactured by themanufacturing process essentially composed of 7 processes mainlyincluding press forming. Accordingly, it is possible to greatly simplifythe manufacturing process so as to reduce the manufacturing cost. Also,it is possible to enhance the accuracy easily. Further, since teethcutting is conducted by means of machining, it is possible toeffectively prevent the occurrence of cracks of the teeth 4 in thequenching process. Furthermore, after the teeth 4 have been quenched,the hardness is increased to a value of 35 to 55 of the Rockwellhardness C. Accordingly, it is possible to ensure a sufficiently highanti-abrasion property of the teeth 4. Also, it is possible to conducttempering on the teeth 4 at a low temperature (for example, at about120° C.) at which the hardness is not lowered.

Next, a quenching test will be explained as follows which was made bythe inventors to find out the optimum quenching method.

In this test, the following test pieces A to L were used, the carboncontents of which are shown on Table

                  TABLE 1                                                         ______________________________________                                                     Carbon                                                                   Test Content                                                                  Piece                                                                              (wt %)                                                           ______________________________________                                                A    0.03                                                                     B    0.04                                                                     C    0.07                                                                     D    0.12                                                                     E    0.13                                                                     F    0.16                                                                     G    0.18                                                                     H    0.24                                                                     I    0.36                                                                     J    0.45                                                                     K    0.47                                                                     L    0.48                                                             ______________________________________                                    

The following 3 types of quenching methods were employed for the test.

(1) Water Jet Method

In the water jet method, the teeth are heated to a temperature not lowerthan the A₃ transformation point. Simultaneously with the stoppage ofheating, the teeth are watered so that the teeth can be quenched.

(2) Water Dipping Method

In the water dipping method, after the teeth have been heated to atemperature not lower than the A₃ transformation point, they are dippedin water.

(3) Oil Dipping Method

In the oil dipping method, after the teeth have been heated to atemperature not lower than the A₃ transformation point, they are dippedin oil.

Values of the Rockwell hardness C of the quenched test pieces weremeasured in the test. Result of the measurement is shown in FIG. 5. Ascan be seen in the diagram shown in FIG. 5, when the test pieces werequenched by the water jet method, the teeth were sufficiently quenchedto a hardness close to the theoretical maximum quenching hardness. Eventhe hardness of the test pieces made of low carbon steel, the carboncontents of which were not less than 0.10 wt %, was not lower than 35 ofthe Rockwell hardness C. Therefore, it can be understood that these testpieces had a sufficiently high anti-abrasion property.

On the other hand, when the test pieces were quenched by the waterdipping method or the oil dipping method, the hardness was not increasedto a value close to the theoretical maximum quenching hardness. In thecase of low carbon steel, the carbon content of which was not more than0.20 wt %, the Rockwell hardness C of the teeth was lower than 35. Thatis, it can be understood that the anti-abrasion property of the teethwas deteriorated.

Although not shown in FIG. 5, except for the above water jet method,there was provided a quenching method in which the test pieces wereheated to a temperature not lower than the A₃ transformation point andwatered by jet immediately before the stoppage of heating. According tothis method, the hardness was somewhat increased more than the hardnessobtained by the water jet method, and it was found that the sameanti-abrasion property as that of the water jet method was positivelyensured. In another case, after the test pieces had been heated to atemperature not lower than the A₃ transformation point, they were dippedin salt water, and then the salt water was agitated with the testpieces. In this case, although the hardness of the test pieces wassomewhat lowered, the same anti-abrasion property as that of the waterjet method was ensured. However, when quenching is conducted in saltwater, the sprocket surface is corroded. Therefore, it is preferable toconduct cooling more quickly in salt water than a case in which coolingis conducted in water.

According to the present invention, the sprocket is made of inexpensivematerial. Therefore, it is possible to greatly reduce the cost ofmanufacturing the sprocket. When the teeth of the sprocket are quenched,the hardness can be increased to a value close to the theoreticalmaximum quenching hardness. Accordingly, the anti-abrasion property ofthe teeth can be greatly enhanced. Further, since the sprocket is madeof low carbon steel, the carbon content of which is low, it is possibleto form a shape of the sprocket from a steel plate mainly by means ofpress forming or only by means of press forming such as fine blanking.Therefore, it is possible to manufacture an accurate sprocket of highquality by a simplified manufacturing process.

In this case, when the carbon content of a low carbon steel plate is setin a range from 0.10 to 0.20 wt %, it is possible to manufacture asprocket from a common inexpensive material while the anti-abrasionproperty of the teeth is maintained at a sufficiently high value. Whenthe teeth are quenched to a hardness of 35 to 55 of the Rockwellhardness C, it is possible to ensure a sufficiently high anti-abrasionproperty of the teeth. Further, when a ratio of the shearing surface tothe inner circumferential surface of the inside diameter mounting holeis set at a value not less than 50%, the assembling accuracy of thesprocket with respect to the rotary shaft can be enhanced.

According to the method of manufacturing a sprocket of the presentinvention, the sprocket is made of an inexpensive low carbon steelplate, the carbon content of which is not more than 0.25 wt %.Consequently, the cost of manufacturing the sprocket can be greatlyreduced. Since the carbon content of material is low, it is possible toform a sprocket from a low carbon steel plate mainly by means of pressforming or only by means of press forming. Accordingly, the number ofprocesses to manufacture the sprocket can be reduced. Further, differentfrom machining, errors caused by the abrasion of a cutting tool can bereduced, and it possible to manufacture an accurate sprocket ofuniformly high quality. Furthermore, chips are seldom generated in themanufacturing process. Therefore, it is easy to process the generatedchips. When the teeth are quickly cooled and quenched from a temperaturenot lower than A₃ transformation point to a temperature not higher than500° C. within 0.5 sec, the Rockwell hardness C of the teeth isincreased to a value close to the theoretical maximum hardness.Accordingly, it is possible to obtain a sprocket having a highanti-abrasion property.

In this case, when the carbon content of a low carbon steel plate is setat 0.10 to 0.20 wt %, it is possible to make a sprocket of a commoninexpensive material while the anti-abrasion property of the teeth ismaintained high. When the teeth are quenched and the hardness isincreased to 35 to 55 of the Rockwell hardness C, it is possible tomaintain the anti-abrasion property to be sufficiently high. When asimple method is employed in which the teeth of the sprocket are heatedto a temperature not lower than the A₃ transformation point and wateredby jet and quenched simultaneously with or immediately after thestoppage of heating, it is possible to quickly cool the teeth from atemperature not lower than the A₃ transformation point to a temperatureof 500° C. within 0.5 sec. The teeth may be watered by jet immediatelybefore the stoppage of heating, so that the teeth can be quenched. Inthis case, the occurrence of a gentle temperature drop caused in aperiod of time from the stoppage of heating to the start of jet-wateringthe teeth is prevented, so that the teeth can be cooled and quenchedmore quickly.

When the teeth of a sprocket are cut by means of machining in themanufacturing process, the occurrence of cracks of the teeth can bepositively prevented, so that the fraction defective can be lowered.When the teeth of a sprocket are heated by a high-frequency inductionheating device in the quenching process, it is possible to quickly heatonly the teeth to a predetermined temperature.

We claim:
 1. A sprocket made of low carbon steel set at 0.10 to 0.20 wt% in the carbon content, comprising:a teeth portion which is quenched toa hardness close to a theoretical maximum quenching hardness, thetheoretical maximum quenching hardness being the maximum Rockwellhardness C value known for a particular carbon content; and an insidediameter mounting hole, wherein said teeth portion is quenched to 35-55in the Rockwell hardness C.
 2. The sprocket of claim 1, wherein a ratioof the shearing surface to the inner circumferential surface of saidinside diameter mounting hole is not less than 50%.
 3. A method ofmanufacturing a sprocket, comprising the steps of:machining a low carbonsteel plate, the carbon content set at 0.10 to 0.20 wt %, into a shapeof a sprocket; and quenching a teeth portion of said machined sprocketby heating to a temperature not lower than the A₃ transformation pointand quickly cooling to a temperature lower than 500° C. within 0.5 secso as to quench said teeth portion to a hardness close to a theoreticalmaximum quenching hardness, the theoretical maximum quenching hardnessbeing the maximum Rockwell hardness C value known for a particularcarbon content. wherein said teeth portion is quenched to 35-55 in theRockwell hardness C.
 4. The method of manufacturing a sprocket of one ofclaim 1, wherein said teeth portion of said machined sprocket is heatedto a temperature not lower than the A₃ transformation point and wateredsimultaneously with or immediately after the stoppage of heating in thequenching process so as to quench said teeth portion.
 5. The method ofmanufacturing a sprocket of claim 3, wherein said teeth portion of saidmachined sprocket is heated to a temperature not lower than the A₃transformation point and watered immediately before the stoppage ofheating in the quenching process so as to quench said teeth portion. 6.The method of manufacturing a sprocket of claim 3, wherein a shape ofsaid sprocket is formed from a low carbon steel plate only by means ofpress forming in the manufacturing process.
 7. The method ofmanufacturing a sprocket of claim 3, wherein at least said teeth portionof said sprocket is machined in the manufacturing process.
 8. The methodof manufacturing a sprocket of claim 3, wherein said teeth portion ofsaid sprocket is heated by a high-frequency induction heating device inthe quenching process.